I agree with everyone here. Red tailed sharks are aggressive fish and a 33 gallon tank isn't going to give him enough room. I think he will only get worse. I would recommend you get rid of the shark.

Also, if you have a 0 Nitrate reading then your tank is probably not cycled. You should have some nitrates in a cycled tank. I wouldn't add anything though until you resolve whatever is going on with that cycle. In fact, the more fish you can remove until you get that settled the better off they will be.

I wouldn't consider 4 fish a school. Someone somewhere came up with the number 6 as a minimum and...well...six isn't really a school either but it is better than 4. So, if you want to keep both the tiger barbs and the gold barbs, then I would say you should get 2 more of each. But, I personally wouldn't do that. I think your tank is rather small for two barb schools. If it were me, I would return the gold barbs and get 8 more tigers. If you want variety, then get 4 albino and 4 regular. Twelve tiger barbs are a lovely site and they are very active and playful. But, again, don't add anything until you are sure your cycle is good.